Vol. 24 No. 3

CHINESE JOURNAL OF GEOCHEMISTRY

2005

Compositional characteristics and geochemical significance of light hydrocarbons for crude oils in Tarim Basin, China* ZHANG Min ( ~

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1 Department of Geochemistry, Yangtze University, Jingzhou 434023, China 2 Key Laboratory of Exploration Technologiesfor Oil and Gas Resources ( Yangtze University), Ministry of Education, Jingzhou 434023, China Abstract Light hydrocarbons, especially C7 isoalkanes, cyclopentanes, cyclohexanes and n-heptane in 540 oils from the Tarim Basin were studied systematically. The results suggested the K 1 values [ (2-MH + 2,3-DMP)/(3-MH + 2,4-DMP) ] of crude oils vary from a lowest value of 0.78, to a highest value of 1.54, but the K l values of the majority of oil samples ( 8 0 % ) range from O. 90 to 1.20. However, the absolute contents of ( 2-methylhcxane + 2,3-dimethylpentane ) and ( 3-methylhexane + 2,4-dimethylpentane) in the total light hydrocarbons ( C4-C7 ) show a similar variation trend. The variation of K 1 values of crude oils can suggest a typical sedimentary environment for source rocks. Therefore, according to the compositional characteristics of CT light hydrocarbons in light of the steady-state catalytic hypothesis, nonmarine oils from the Yingmaili area and marine oils from the Tazhong uplift in the Tarim Basin were genetically classified.

Key words

Mango' s light hydrocarbon parameter ( K 1 ) ; geochemical characteristic ; crude oil; Tar-

im Basin

1 Introduction Light hydrocarbon ( L H ) is an important component in petroleum and natural gas, which accounts for over 50% of the carbon in petroleum ( Mango, 1997). But the origin of LH is not clear. In general, light hydrocarbons ( L H s ) are conventionally regarded as the thermolytic products from the progressive thermal breakdown of kerogen and oil (Tissot and Wehe, 1984). A ratio of the sums of isoheptanes is constant over the course of petroleum generation, and a parentdaughter steady-state catalytic process is proposed for the origin of light hydrocarbons in petroleum (Mango, 1987, 1990a, b, 1996; ten Haven, 1996). This paper focuses on the geochemical characteristics of C 7 light hydrocarbons in 540 oil samples from the Tarim Basin, based on the steady-state catalytic hypothesis of the origin of LHs.

2 Samples and analytical methods The Tarim Basin covering an area of 560000 km 2 , was a platform during the Paleozoic and then developed ISSN 1000-9426 * This study was financially supported by the Natural Science Foundation of Hubei Province ( No. 2004ABA144).

into a Meso-Cenozoic basin. There are two marine facies source rocks, i. e . , Cambrian-Early Ordovician carbonates and Mid-Late Ordovician argillaceous limestones and limestones. In addition, the Triassic-Jurassic strata contain good quality oil source rocks of lacustrine facies (Zhang Min et al., 1997, 2004). The database has been constructed in terms of the analyses of 540 petroleum samples from the Tarim Basin. Oils were analyzed by high-resolution gas chromatography. Samples were collected from all stratigraphic units, from Early Paleozoic to Late Cenozoic. The oils were collected by field technicians from the wellheads or separator units of producing oil wells. These crude oils in the petroleum system can be classified as three groups: marine oils (369 samples), laustrine oils (134 samples) and coal-generated oils (37 samples). The light hydrocarbons were analyzed using Varian 3400 gas chromatograph equipped with a capillary column (OV101 50 m x 0 . 2 5 ram) with N 2 as the cartier gas. The initial temperature was held at 30°C for 7 min and then programmed to 120°C at a rate of 3°(:/ rain and held at 120°C for 20 min.

3 Results and discussion 3. 1 Compositional characteristics of light hydrocarbon compounds There are four kinds of light hydrocarbons, i.e. ,

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Compositional characteristics and geochemical significance

normal alkanes, isoalkanes, cycloalkanes and aromatic hydrocarbons based on the structure type of their compounds. The relative abundances of the four kinds of compounds in Tafim oils are different. Light hydrocarbons, especially C7 isoalkanes, dimethylcyclopentanes, methylcyclohexanes and n-heptane were detected in 540 oil samples from the Tarim Basin. The contents of dimethylcyclopentanes in oils are of no variation, but those of isoalkanes and methylcyclohexanes are remarkably variable. The results indicated the contents of methylcyclohexane in oils from marine source rocks are lower by 3 5 % , those of methylcyclohexane in oils from non-marine source rocks are higher by 3 5 % , with the maximum up to 70%. So the compositional characteristics of C7 light hydrocarbon compounds reflect the differences in type of organic matter in different source rocks. Figure 1 shows a nearly constant ratio between the positional isomers, 2-MH and 3-MH, while Fig. 2 and Fig. 3 display an increasing variation trend for similar non-isomers within a class (2-MP/2-MH) and similar non-isomers between classes ( M C P / M C H ) , respectively. Although the amounts of LHs vary from oil to oil, and the relative contents of the different structural classes in Tarim oils vary, the relative amounts of LHs within the same structural class maintain remarkably constant. For example, ratios of similar isoalkanes ( e. g. 2-MP/3-MP) , cyclopentanes ( e. g. CP/MCP) and cyclohexanes (e. g. CH/MCH) are nearly constant, while those between the classes, even though similar (e. g. MCP/MCH) , are much lower.

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It was suggested that nearly 20 years ago the over-

all compositional mix of light isoparaffins in petroleum was constant ( Mango, 1987 ) , but the origin of light hydrocarbons in petroleum is not clear. Isoheptanes in 2258 oil samples collected from various parts of the world, but mainly from North America, display a striking proportionality among the four isomers: 2-methylhexane ( 2 - M H ) , 3-methylhexane ( 3 - M H ) , 2, 3-dimethylpentane ( 2 , 3-DMP), and 2, 4-dimethylpentane (2, 4-DMP). Regardless of their different absolute concentrations in oils, their relative amounts are such that the K 1 ratio remains approximately one (i. e. , K 1 ~--1 ) [ ( 2 - n i l + 2, 3-DMP) / ( 3-MH + 2, 4-DMP) 1 ] ( Mango, 1987). Figure 4 shows a cross-plot of [ 2-methylhexane + 2, 3-dimethylpentane ] and [ 3-methylhexane + 2, 4dimethylpentane ] as % total light hydrocarbons in 540 crude oils from the Tarim Basin. The results indicated

230

ZHANG Min et al.

there are strong linear relations among the four isomers of isoheptanes in petroleum. The K 1 ratios in crude oils (540 oil samples) in the Tarim Basin vary from a lowest value of 0 . 7 8 to a highest value of 1 . 5 4 , but the K 1 ratios in the majority of oil samples ( 8 0 % ) range from 0 . 9 0 to 1.20. However, the K 1 ratios in 5% of oil samples are lower than 0 . 9 ; at the same time, the K 1 ratios in 15% o f oil samples are higher than 1.20. These oils have K1 values ranging from 1.20 - 1 . 5 4 , in the Tazhong oilfield of the Tarim Basin, the K 1 ratios in 84 oil samples have a mean of 1.38 with a standard deviation of 0. 096. The characteristics reflect the oils from the Tazhong uplift sourced mainly from sulfurrich kerogens (Zhang Min and Zhang Jun, 1999; Zhang Chunming et al. , 1998).

Vol. 24

im Basin have been genetically classified (Figs. 5 and 6). 40.00 + Marine oil [] Non-marine oil

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Variations in K 1 of crude oils suggest a typical sedimentary environment for source rocks (Zhang Min and Zhang Jan, 1999, Xiao Yitian and James, 1997; Obermajer et al. , 2002). Therefore, according to the compositional characteristics of C7 light hydrocarbons in the steady-state catalytic hypothesis ( Mango, 1 9 9 4 ) , based on the Parent-Daughter plot of P2 (2-methylhexane + 3-methylhexane), as % C7 light hydrocarbons, and a ratio of P3 ( sum of dimethylpentanes ) / N 2 ( 1,1dimethylcyclopentane + 1 t3-dimethylcyclopentane), as well as the Parent-Daughter plot of n-heptane, as % C 7 light hydrocarbons, and a ratio of CPs ( 1 c2-dimethylcyclopentane + lt2-dimethylcyelopentane ) / C H s ( methylcyclohexane), laustrine oils from the Yingmaili area and marine oils from the Tazhong uplift in the Tar-

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CPsJCHs Fig. 6. Plot of nC7 ( % ) vs. ratio of CPs/CHs for Yingmaili laustrine oils and Tazhong uplift marine oils in the Tarin Basin. nC7% = nC7 x 100%/(C4-C 7 ) ; CPs = 1, 2-DMCP (c, t); CHs =MCH.

4 Conclusions ( 1 ) Although the relative contents of light hydro-

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Compositional characteristics and geochemical significance

carbons of different structural classes in 540 Tarim oils vary, the relative amounts of LHs of the same structural class remain remarkably constant. ( 2 ) The K 1 ratios in the majority of oil samples ( 80% ) from 540 Tarim oils range from O. 90 to 1.20. However, the K 1 ratios in the other oil samples, especially those from the Tazhong oilfield range from 1.20 -1.54. ( 3 ) According to the compositional characteristics of C7 light hydrocarbons in the steady-state catalytic hypothesis, some new geochemical parameters including Pz/N2 and CPs/CHs have been suggested. References Mango F.D. (1987) An invariance in the isoheptanes of petroleum [ J]. Sc/ence. 2 7 3 , 5 1 4 - 5 1 7 . Mango F.D. (1990a) The origin of light cycloalkanes in petroleum [ J]. Geochim. Cosmochim. Acta. 54, 2 3 - 2 7 . Mango F.D. (1990b) The origin of light hydrocarbons in petroleum: A kinetic test of steady-state catalytic hypothesis [ J ] . Geochim. Cosmochim. Acta. 54, 1315-1323. Mango F.D. ( 1994 ) The origin of light hydmcarbans in petroleum: Ring preference in the closure of carbocyclic rings [ J]. Geochim. Cosmochim. Acta. 58, 8 9 5 - 9 0 1 . Mango F.D. (1996) Transition metal catalysis in the generation of natural gas [ j ] . Org. Geochem. 2 4 , 9 7 7 - 9 8 4 .

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Mango F.D. (1997) The light hydrocarbons in petroleum: A critical review [ J ] . Org. Geochem. 2 6 , 4 1 7 -440. Obermajer M. , Osadete K. G. , and Fowler M. G. (2002) Delineating compositional variabilities among crude oils from Central Montane, USA, using light hydrocarbon and biomarker characteristics [ J ]. Org. Geochem. 33, 1333 - 1359. ten Haven H.L. (1996) Application and limitations of Mango' s light hydrocarbon parameters in petroleum correlation studies [ J ]. Org. Geochem. 2 4 , 9 5 7 - 976. Tissot B.P. and Wehe D.H. ( 1984 ) Petroleum Formation and Occurrence [M]. pp. 215-220. Springer, Berlin. Xiao Yitian and James A.T. (1997) Is Acid Catalyzed lsomerization Respoasible for the Invariance in the Isoheptanes of Petroleum [ C ]. pp. 796 - 7 9 7 . 18th International Meeting on Organic Geochemistry (abstracts). Zhang Chunming, Zhao Honjing, and Mei Bowen (1998) The variance of Mango' s light-hydrocarbon parameter K [ J ]. Chinese Journal of Geochemistry. 17, 148 - 151. Zhang Min, Lin R. , and Mei B. ( 1997 ) Reservoir Geochemistry: Approach to Kuche Petroleum System of Tarim Basin, China [ M]. pp. 20 -58. Changqing University Press, Chongqing (in Chinese). Zhang Min and Zhang Jun ( 1999 ) Composition characteristics and geological significance of thiophene-type compounds for crude oils in Tarim Basin, China [ J ] . Acta Sedimentologica Sinica. 17, 121 126 ( in Chinese with English abstract). Zhang Min, Zhang Jun, Zhao Hangjing, and Feng Min (2004) Migration fractionation of neutral nitrogen compounds of crude oils from Tabei field in the Tarim Basin, China [ J ] . Chinese Journal of Geochemistry. 22, 8 9 - 9 3 .